1. Field of the Invention
The present invention relates to a device for dynamic measurement of the surface tension at boundary surfaces between liquids and gasses according to the method of evaluation of the maximum bubble pressure.
2. Description of the Related Art
A device of this type and a measurement process for this is described in greater detail in, for example, EP 0 149 500 A2.
For the theory and for further understanding of the measurement method, reference is made to the company publication: Blasendrucktensiometer BP2, Benutzerhandbuch, Kruss GmbH, Hamburg 1995.
The mentioned devices are suitable for stationary laboratory testing of liquids. Very high demands are made on the measurement conditions. Thus, for example, the state of fullness or, rather, the immersion depth of the capillary in the measurement liquid must be adjusted with millimeter precision with the aid of a sinking device, in which process a height adjustment up to the "jumping" of the liquid onto the capillary must first be undertaken manually and with very great care. The installation location is to be chosen with great care, since, for example, vibrations due to personnel walking by or drafts ensure a serious adulteration of the measurement results. Also, the measurement apparatus is to be carefully aligned. By reason of the demanding design of the measurement place and the high weight of about 20 kg, mobile application is prohibited. A further reason for the fixed design of the measurement place is that for the generation of bubbles relatively high pressures are required, for which, in turn, large external pressure-gas producers must be available and, obviously, an electrical connection must be present. The regulation of the bubble formation and/or bubble frequency takes place in a demanding manner by means of analog valves. The evaluation can take place only on a personal computer.
Furthermore, devices are known that for the purpose of continuous measurements are in constant contact with a particular liquid to be tested (DE 41 12 417 A1, DE 43 03 133 A1), and which are very expensive. An external air compressor, two compressed-air hoses, two valves, and two different precision capillaries, immersed to exactly the same depth, are necessary for the generation of the gas bubbles, as well as a pressure-difference gauge at the capillary feeds. The evaluation takes place on a personal computer. Disturbances, caused by a bubble break-off at one of the capillaries, makes an evaluation difficult.
In all known devices the pressure or rather the pressure difference between two capillaries is measured as an absolute value, for which purpose relatively cost-intensive pressure sensors with a very precise calibration are required.
Finally, from DE 44 23 720 C1 is known a generically different device for measuring the surface tension of preferably molten metals, with one capillary for the gas supply, which capillary is conducted for example vertically through the bottom of a crucible receiving the molten metal and terminates at a nozzle for the formation of the gas bubbles. With knowledge of the surface tension of cast iron, by means of this device conclusions can be made regarding the graphite morphology of the carbon contained in the cast iron, and the sulfur content of the pig iron or even the refinement processing of aluminum-silicon alloys can be judged.
It is a matter here of an elaborate apparatus to be operated in a stationary manner, with which apparatus the frequency of the gas bubbles emerging from the nozzle into the molten cast iron is determined. The relatively long capillary with an inner diameter of only 0.7 to 1.5 mm causes a considerable flow resistance for the gas the thus necessitates a high expenditure of energy during the measuring process. Beyond that, during the operation the moistening characteristics and the inner diameter of the capillary are altered by the measurement melt penetrating more or less into the capillary, and thus the measurement parameters are also altered uncontrollably, which leads, finally, to uncertain measurement results. Moreover, after every measurement a careful purging of the capillary or, better, its replacement is necessary. Both of these are time-consuming and expensive.